Television receivers of very thin thicknesses, such as a liquid crystal display device (LCD) or a plasma display panel (PDP), have been devised and put to practical use to take the place of the cathode ray tube (CRT) which has long been used since the start of television broadcasting. In particular, the liquid crystal display device, employing a liquid crystal display panel, is a highly promising display type and is expected to become widespread in time to come because it permits driving at low power usage and a large size liquid crystal panel may be offered at a reasonable cost.
The mainstream of the liquid crystal display device is that of the backlight system in which a light transmitting liquid crystal display panel, fitted with a color filter, is illuminated from its back side with a backlight device. As a light source for the backlight device, a fluorescent lamp, radiating white light using a fluorescent tube (Cold Cathode Fluorescent Lamp), has predominantly been in use.
On the other hand, in consideration that the CCFL uses silver contained in the fluorescent tube and hence may be pollutant to the environment, a light emitting diode (LED) is thought to be auspicious as a light source to be used in place of the CCFL. With the development of the blue light emitting diode, the three sorts of the light emitting diodes, radiating the light for three prime colors, namely green light, red light and blue light, are now in order, and white light of high color purity may be obtained by mixing the green light, red light and blue light, radiated by these three sorts of the light emitting diodes. With use of these light emitting diodes, as the light source of the backlight device, the color light beams, obtained from the liquid crystal display panel, may be improved in color purity, thereby appreciably broadening the gamut of color reproduction as compared to the case of using the CCFL.
Patent Publication 1 Japanese Patent Application laid-Open No. H07-191311
FIG. 1 shows a backlight box 50, as a backlight device employing a fluorescent lamp, as a light source. FIG. 1A depicts a front view of the backlight box 50 and FIG. 1B depicts a cross-sectional view of the backlight box 50 taken along line X-X of FIG. 1A. Meanwhile, a light transmitting/diffusing plate 53, shown in FIG. 1B, is not shown in FIG. 1A for illustrating how the fluorescent lamps 1 are arrayed.
Referring to FIG. 1, the backlight box 50 includes a casing 50a, opened at an upper end, a plural number of fluorescent lamps 51, arranged parallel to one another in the casing, and a light transmitting diffusion plate 53, provided for closing the opened end of the casing 50a. On the inner surfaces (inner lateral surfaces and inner bottom surface) of the casing 50a, there are formed reflecting surfaces 52 for scattering and reflecting the white light emitted from the fluorescent lamps 51.
On top of the light transmitting diffusion plate 53 of the backlight box 50, a light transmitting liquid crystal display panel, not shown, is provided to form a liquid crystal display device. The light transmitting diffusion plate 53, provided to the backlight box 50, diffuses the direct light from the fluorescent lamps 51 or light reflected from the reflecting surfaces 52, to generate white light of uniform light volume free from variations in color or in brightness. The so generated white light is radiated by surface light emission. The backlight box 50 illuminates the liquid crystal display panel with the white light radiated by surface light emission.
Specifically, the light transmitting diffusion plate 53 is subjected to processing for enlarging the angle of diffusion of incident light, or to preset patterning, such as to prevent variations in brightness from being produced to produce irregular brightness due to formation from the direct light emitted by the fluorescent lamps 51 of blurred plural images of a light source by the plural fluorescent lamps 51 on the light transmitting/diffusing plate 53. The light transmitting/diffusing plate 53 may also be increased in thickness for enhancing the effect of diffusion.
However, even if ideal perfect scattering is realized by the light transmitting diffusion plate 53, as a result of the aforementioned processing for adjusting the angle of diffusion or for patterning, there still persists the problem that an image of a light source on the light transmitting diffusion plate 53 cannot completely be erased, in case the distance between the fluorescent lamps 51, emitting the white light, and the light transmitting diffusion plate 53, is short, that is, if the backlight box 50 is not of a sufficient thickness.
Moreover, if the light transmitting diffusion plate 53 is of an enlarged angle of diffusion, or is subjected to preset patterning, as described above, there may be raised a further problem that the light volume of the component in the vertical direction of the white light from the backlight box 50 with respect to the liquid crystal display panel is excessively attenuated, thus drastically lowering the front-surface brightness.
FIG. 2 shows a backlight box 60, as a backlight device employing a fluorescent lamp, as a light source. FIG. 2A depicts a front view of the backlight box 60 and FIG. 2B depicts a cross-sectional view of the backlight box 60 taken along line X-X of FIG. 2A. Meanwhile, a light transmitting/diffusing plate 63, shown in FIG. 2B, is not shown in FIG. 2A for illustrating how red light emitting diodes 61R, green light emitting diodes 61G and blue light emitting diodes 61B are arrayed.
Referring to FIG. 2, the backlight box 60 includes, within a casing 60a, opened at an upper end, a plural number of the red light emitting diodes 61R, a plural number of the green light emitting diodes 61G and a plural number of the blue light emitting diodes 61B, arranged at a preset sequence and with preset separation from one another. In an opened end of the casing 60a, the light transmitting/diffusing plate 63, having the same function as that of the backlight box 60, is mounted. On inner surfaces (inner lateral surfaces and inner bottom surface) of the casing 60a, there are provided reflecting surfaces 62 for scattering and reflecting red light, green light and blue light, radiated from the red light emitting diodes 61R, green light emitting diodes 61G and the blue light emitting diodes 61B, in the inside of the casing 60a, respectively. Meanwhile, if, in the explanation to follow, the red light emitting diodes 61R, green light emitting diodes 61G and the blue light emitting diodes 61B, are referred to collectively, they are simply referred to as light emitting diodes 61.
A liquid crystal display device is architected by arranging a light transmitting liquid crystal display panel, not shown, on the light transmitting/diffusing plate 63 of the backlight box 60. The backlight box 60 mixes the red light, green light and the blue light, emitted from the light emitting diodes 61, by reflection by the reflecting surfaces 52 and by diffusion by the light transmitting diffusion plate 63, into white light, which is then radiated to the liquid crystal display panel, not shown, by surface light emission and light illumination.
Referring to FIG. 2A, light emitting diodes 61 are arrayed in the casing 60a of the backlight box 60 at a preset spacing from one another. The light emitting diodes 61, thus arrayed in the casing 60a of the backlight box 60, operate as point light sources, and hence the light beams of the respective colors are not emitted from the same points. It is therefore extremely difficult to mix the colors evenly on the light transmitting/diffusing plate 63 such as to preclude variations in color or in brightness.
For example, as shown in FIG. 3B, when the casing 60a of the backlight box 60 is reduced in thickness, such that a sufficient distance from the light emitting diodes 61 as far as the light transmitting/diffusing plate 63 may not be obtained, the light beams of the respective colors, getting to the light transmitting/diffusing plate 63, exhibit the light distribution shown in FIG. 3A, with the consequence that sufficient color mixing cannot be achieved.
FIG. 3A depicts a front view of the backlight box 60, showing the state of light distribution of the light emitting diodes 61, and FIG. 3B is a cross-sectional view of the backlight box 60, taken along line X-X in FIG. 3A. Meanwhile, the light transmitting diffusion plate 63 is not shown in FIG. 3A for illustrating the state of light distribution of red, green and blue light beams, radiated from the light emitting diodes 61.
For eliminating variations in color and brightness of the white light, obtained on color mixing by the light transmitting/diffusing plate 63, the light beams of the respective colors, getting to the light transmitting/diffusing plate 63 from the light emitting diodes 61, need to be superposed together sufficiently, as shown in FIG. 4A. This is made possible by sufficiently increasing the thickness of the casing 60a to provide for a sufficient distance between the light emitting diodes 61 and the light transmitting/diffusing plate 63, as shown in FIG. 4B. Thus, the backlight box 60, employing the light emitting diodes 61 as the light source, is of a marked thickness, even as compared with the backlight box 50 employing the fluorescent lamps 51 as the light source, because the casing 60a has to be of an increased thickness. Hence, the backlight box 60, employing the light emitting diodes 61 as the light source, suffers from the problem that it is further deterrent to reducing the thickness of the liquid crystal device.